Light-induced aluminum plating on silicon for solar cell metallization

ABSTRACT

Methods for light-induced electroplating of aluminum are disclosed herein. Exemplary methods may comprise preparing an ionic liquid comprising aluminum chloride (AlCl 3 ) and an organic halide, placing the silicon substrate into the ionic liquid, illuminating the silicon substrate, the illumination passing through the ionic liquid, and depositing aluminum onto the silicon substrate via a light-induced electroplating process, wherein the light-induced electroplating process utilizes an applied current that does not exceed a photo-generated current generated by the illumination.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 16/113,822 filed onAug. 27, 2018, now U.S. Patent Application Publication No. 2019-0067498entitled “LIGHT-INDUCED ALUMINUM PLATING ON SILICON FOR SOLAR CELLMETALLIZATION.” U.S. Ser. No. 16/113,822 claims priority to, and thebenefit of, U.S. Provisional Patent Application No. 62/551,037, filed onAug. 28, 2017, entitled “LIGHT-INDUCED ALUMINUM PLATING ON SILICON FORSOLAR CELL METALLIZATION.”

This application is also a continuation-in-part of U.S. Ser. No.16/432,702 filed on Jun. 5, 2019, now U.S. Patent ApplicationPublication No. 2019-0312162 entitled “SOLAR CELLS FORMED VIA ALUMINUMELECTROPLATING.” U.S. Ser. No. 16/432,702 is a division of U.S. Ser. No.15/079,359 filed on Mar. 24, 2016, now U.S. Patent ApplicationPublication No. 2016-0204289 entitled “SOLAR CELLS FORMED VIA ALUMINUMELECTROPLATING.” U.S. Ser. No. 15/079,359 is a continuation of PCTSerial No. PCT/US2014/067338 filed on Nov. 25, 2014, now WIPOPublication WO 2015-081077 entitled “SOLAR CELLS FORMED VIA ALUMINUMELECTROPLATING.” PCT Serial No. PCT/US2014/067338 claims priority to,and the benefit of: (i) U.S. Provisional Patent Application No.62/055,378 filed on Sep. 25, 2014 and entitled “SOLAR CELLS FORMED VIAALUMINUM ELECTROPLATING;” (ii) U.S. Provisional Patent Application No.62/018,320 filed on Jun. 27, 2014 and entitled “ALUMINUM ELECTROPLATINGOF SOLAR CELLS;” and (iii) U.S. Provisional Patent Application No.61/908,824 filed on Nov. 26, 2013 and entitled “SILICON PHOTOVOLTAICSOLAR CELLS WITH ELECTROPLATED ALUMINUM ELECTRODES.”

Each of the foregoing applications are hereby incorporated by reference,including but not limited to those portions that specifically appearhereinafter, but except for any subject matter disclaimers ordisavowals, and except to the extent that the incorporated material isinconsistent with the express disclosure herein, in which case thelanguage in this disclosure shall control.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under 1336297 awarded bythe National Science Foundation. The Government has certain rights inthe invention.

TECHNICAL FIELD

The present disclosure relates to materials deposition, and inparticular to electroplating of aluminum in connection with siliconsolar cells.

BACKGROUND

Increasing expense and limited supply of silver has generated interestin alternative materials for use in connection with electrodes insilicon solar cells. However, common alternatives, such as copper,suffer from various drawbacks, for example a need for barrier layers,protective layers, and so forth. Accordingly, improved solar cells andmethods related to fabrication of the same remain desirable.

SUMMARY

A method for light-induced electroplating of aluminum directly onto asilicon substrate is disclosed herein. In various embodiments, themethod comprises preparing an ionic liquid comprising aluminum chloride(AlCl₃) and an organic halide, placing the silicon substrate into theionic liquid, illuminating the silicon substrate, the illuminationpassing through the ionic liquid, and depositing aluminum onto thesilicon substrate via a light-induced electroplating process, whereinthe light-induced electroplating process utilizes an applied currentthat does not exceed a photo-generated current generated by theillumination.

In various embodiments, the method further comprises cleaning thesilicon substrate with at least one of hydrogen fluoride, hydrogenchloride, hydrogen peroxide, sodium hydroxide, potassium hydroxide, orammonium hydroxide. In various embodiments, the method further comprisespatterning a partially-processed silicon solar cell to expose thesilicon substrate. In various embodiments, the patterning comprises atleast one of laser ablation or lithography. In various embodiments, themethod further comprises cleaning the deposited aluminum with deionizedwater. In various embodiments, the method further comprises annealingthe deposited aluminum and the silicon substrate to reduce a resistivityof the deposited aluminum. In various embodiments, the organic halide is1-ethyl-3-methylimidazolium tetrachloraluminate (EMIm-AlCl₄). In variousembodiments, the light-induced electroplating process utilizes atwo-electrode electrolyzer. In various embodiments, the two-electrodeelectrolyzer, an anode comprises an aluminum wire mesh, and a cathodecomprises the silicon substrate. In various embodiments, thelight-induced electroplating process comprises applying a voltagebetween the anode and the cathode to achieve a current of between 5milliamps per centimeter squared and 50 milliamps per centimetersquared. In various embodiments, the depositing is performed with theionic liquid at a temperature of between 20 degrees Celsius and 150degrees Celsius. In various embodiments, the depositing is performedwith the ionic liquid at a temperature of about 100 degrees Celsius orgreater. In various embodiments, the depositing occurs in an inertambient atmosphere. In various embodiments, the illumination comprises awavelength of between 600 nanometers and 1000 nanometers. In variousembodiments, the ionic liquid is disposed in a container having atransparent bottom, and wherein the illumination is provided by lightemitting diodes disposed below the bottom of the container.

An n-type back-emitter solar cell is disclosed herein, the solar cellcomprising a front finger electrode comprising aluminum and formed bylight-induced electroplating of aluminum onto silicon over a patternedsilicon nitride layer, and a back electrode comprising aluminum andformed by screen printing, wherein an electrical contact between thefront finger electrode and a silicon substrate of the solar cell isformed by annealing at a temperature between 100 degrees Celsius and 500degrees Celsius.

In various embodiments, the light-induced electroplating of aluminumonto silicon is performed at a temperature of between 20 degrees Celsiusand 150 degrees Celsius. In various embodiments, the solar cell isconfigured with efficiency above 15%. In various embodiments, the solarcell further comprises a zinc capping layer.

A method for processing a silicon solar cell comprises preparing anionic liquid comprising aluminum chloride (AlCl₃) and1-ethyl-3-methylimidazolium tetrachloraluminate (EMIm-AlCl₄), patterninga partially-processed silicon solar cell to expose an n-type surface ofa silicon substrate, cleaning the n-type surface with at least one ofhydrogen fluoride, hydrogen chloride, hydrogen peroxide, sodiumhydroxide, potassium hydroxide, or ammonium hydroxide, bringing then-type surface into contact with the ionic liquid; illuminating n-typesurface, wherein the illumination passes through the ionic liquid andcomprises a wavelength between about 600 nanometers and 1000 nanometers,depositing aluminum onto the silicon substrate via a light-inducedelectroplating process, wherein the light-induced electroplating processcomprises applying a current between an aluminum back electrode of thepartially-processed silicon solar cell and an aluminum mesh disposed inthe ionic liquid, cleaning the deposited aluminum with deionized water,and annealing the deposited aluminum and the n-type surface to reducethe resistivity of the electroplated aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the following description and accompanying drawings:

FIGS. 1a and 1b illustrate electrodes for silicon solar cells known inthe prior art;

FIG. 1c illustrates a light-induced aluminum electrode in accordancewith an exemplary embodiment;

FIGS. 2a and 2b illustrate exemplary systems for light-induced aluminumplating on silicon solar cells in accordance with exemplary embodiments;

FIG. 3 illustrates light-induced aluminum plating on a silicon solarcell in accordance with an exemplary embodiment;

FIG. 4 illustrates a transmission spectrum of a prepared ionic liquidfor light-induced aluminum plating in accordance with an exemplaryembodiment;

FIG. 5 illustrates external quantum efficiency of a partially-processedsilicon solar cell in accordance with an exemplary embodiment;

FIG. 6 illustrates a combined absorption spectrum of apartially-processed silicon solar cell in a prepared ionic liquid inaccordance with an exemplary embodiment;

FIG. 7a is a scanning electron microscopy image of light-inducedaluminum on an n-type silicon solar cell with a back electrode, inaccordance with an exemplary embodiment;

FIG. 7b is a scanning electron microscopy image of light-inducedaluminum plated at 25° C. on an n-type silicon solar cell with a backelectrode, in accordance with an exemplary embodiment;

FIG. 7c is a scanning electron microscopy images of light-inducedaluminum plated at 60° C. on an n-type silicon solar cell with a backelectrode, in accordance with an exemplary embodiment;

FIG. 8a illustrates an energy dispersive x-ray spectroscopy spectrum oflight-induced aluminum on an n-type silicon solar cell with a backemitter, in accordance with an exemplary embodiment;

FIG. 8b illustrates an energy dispersive x-ray spectroscopy spectrum oflight-induced aluminum plated at 25° C. on an n-type silicon solar cellwith a back electrode, in accordance with an exemplary embodiment;

FIG. 8c illustrates thickness of light-induced aluminum deposits as afunction of plating temperature;

FIG. 8d illustrates resistivity of light-induced aluminum deposits as afunction of plating temperature;

FIG. 9 illustrates a p-type aluminum back-surface field silicon solarcell structure with a light-induced aluminum front electrode inaccordance with an exemplary embodiment;

FIG. 10 illustrates a n-type back-emitter silicon solar cell structurewith a light-induced aluminum front electrode in accordance with anexemplary embodiment;

FIG. 11 illustrates a p-type passivated emitter rear contact siliconsolar cell structure with an light-induced aluminum front electrode inaccordance with an exemplary embodiment; and

FIG. 12 illustrates a method of light-induced aluminum plating on asilicon nitride layer of an n-type silicon solar cell with a backelectrode, in accordance with an exemplary embodiment.

DETAILED DESCRIPTION

The following description is of various exemplary embodiments only, andis not intended to limit the scope, applicability or configuration ofthe present disclosure in any way. Rather, the following description isintended to provide a convenient illustration for implementing variousembodiments including the best mode. As will become apparent, variouschanges may be made in the function and arrangement of the elementsdescribed in these embodiments without departing from principles of thepresent disclosure.

For the sake of brevity, conventional techniques for materialsdeposition, electroplating, silicon solar cell fabrication, and the likemay not be described in detail herein. Furthermore, the connecting linesshown in various figures contained herein are intended to representexemplary functional relationships and/or physical couplings betweenvarious elements. It should be noted that many alternative or additionalfunctional relationships or physical connections may be present in apractical silicon solar cell and/or method for electroplating ofaluminum.

Prior solar cells suffer from various deficiencies. For example, manysolar cells utilize silver for an electrode or electrodes. However,silver is increasing in expense and decreasing in availability. A commonalternative, copper, requires barrier layers and/or protective layers,increasing complexity and cost. In contrast, these and othershortcomings of prior approaches may be overcome by utilizing principlesof the present disclosure, for example as illustrated in variousexemplary embodiments. For example, by utilizing light-inducedelectroplating of aluminum, silicon solar cells having acceptableperformance and reduced cost may be achieved.

Light-induced aluminum plating as disclosed herein allows directdeposition of aluminum on highly-resistive silicon, thus eliminating theneed for a seed layer between aluminum and silicon. As compared tocopper plating, it simplifies the aluminum plating process for siliconsolar cells and reduces the manufacturing cost of aluminum plating.Exemplary silicon solar cells which are compatible with thelight-induced aluminum plating process are disclosed herein.

Silver is commonly used as the front finger electrode in today's siliconsolar cells. In 2016, about 2,600 tonnes of silver were consumed for theproduction of about 73 GWp silicon solar cells. Though silver hasseveral advantages as the front electrode in silicon solar cells, thereare two major issues with future prospects of silver in silicon solarcells. The first issue is the high price of silver. Currently, the costof silver in silicon cells is about $0.022/Wp while the price of thesolar cell is about $0.20/Wp, meaning that silver contributes to about10% of the cost of a silicon solar cell. The second issue is the limitedreserve of silver on this planet. Per the U.S. Geological Survey, theglobal known reserve of silver is 570,000 tonnes. Based on the silverconsumption to solar cell production ratio of 2016, the silver reservewould allow the production of 16 TWp of silicon solar cells if all thesilver reserve were exclusively used for silicon solar cell production.16 TWp of solar cells would meet only about 8% of the projected globalenergy demand in 2040.

Technologies are being developed to replace silver in silicon solarcells with an Earth-abundant, low-cost, and low-resistivity metal. Thecandidate metals which meet these requirements include copper andaluminum. However, there is a major disadvantage for copper as the frontelectrode in silicon solar cells. Copper is detrimental to the minoritycarrier lifetime in silicon and can significantly reduce the efficiencyof the solar cell, so it cannot be deposited directly on silicon. Abarrier layer, typically made of electroplated nickel, is requiredbetween copper and silicon to prevent copper from touching silicon.

Principles of the present disclosure may be utilized in connection withprinciples disclosed in U.S. patent application Ser. No. 15/079,359filed on Mar. 24, 2016, now U.S. Patent Application Publication No.2016-0204289 entitled “SOLAR CELLS FORMED VIA ALUMINUM ELECTROPLATING”,the contents of which are hereby incorporated by reference in theirentirety for all purposes, and which disclose an electroplating processof aluminum to replace silver in silicon solar cells. Aluminum is benignto silicon and has been used in direct contact with silicon in solarcells for decades. Although no barrier layer is needed between aluminumand silicon, a seed layer, typically made of electroplated nickel, isstill required to facilitate conventional aluminum plating on silicondue to the highly resistive silicon.

In accordance with principles of the present disclosure, a newelectroplating process and associated equipment for aluminum isdisclosed, i.e., light-induced aluminum plating. Light-induced platinghas not previously been explored for aluminum deposition on siliconsolar cells. Because the plating current is photo-generated across theentire solar cell, there is no need for a seed layer in light-inducedplating. FIG. 1a illustrates a plated copper electrode 200 as known inthe prior art, including n-type emitter 102, p-type base 104,electroplated nickel barrier layer 106, copper front electrode 108, tincapping layer 110, back surface field 112, and back electrode 114. FIG.1b illustrates a conventional plated aluminum electrode 300 as known inthe prior art, including n-type emitter 102, p-type base 104,electroplated nickel seed layer 107, aluminum front electrode 116, zinccapping layer 118, back surface field 112, and back electrode 114. FIG.1c illustrates a light-induced aluminum electrode 400 in accordance withprinciples of the present disclosure. In various embodiments,light-induced aluminum electrode 400 comprises n-type emitter 102,p-type base 104, direct plated aluminum front finger electrode 120, zinccapping layer 118, back surface field 112, and back electrode 114, amongother elements.

Without a barrier or seed layer, principles of the present disclosuresimplify the conventional aluminum plating process for silicon solarcells, and also reduce the manufacturing cost of aluminum plating ascompared to copper plating. In addition, several silicon solar cellswhich are compatible with the light-induced aluminum plating process aredescribed.

The back electrode in certain exemplary solar cells disclosed herein ismade of screen-printed aluminum and the front electrode is made oflight-induced aluminum, i.e., no silver is used as an electrode in thesesolar cells. In various embodiments, the ionic liquid for light-inducedaluminum plating comprises aluminum. In various embodiments, the lightsource is of any wavelength which is transparent in the ionic liquid butis absorbed by silicon. The light-induced plating process may be carriedout in air at a temperature slightly above room temperature but below200° C., or in an inert gas at a temperature between room temperatureand 200° C., or in vacuum at a temperature between room temperature and200° C. In various embodiments, the light-induced aluminum electrode isin direct contact with a silicon substrate, for example, the frontn-type silicon emitter of the solar cell.

Light-Induced Plating of Aluminum on Silicon

An important principle of the present disclosure is a method oflight-induced plating of aluminum on silicon solar cells as disclosedherein. In an exemplary embodiment, a light-induced aluminum platingsystem 500, as shown in FIG. 2, includes several components: an ionicliquid 530, a light source 504, a heat source 506, a direct-currentpower supply 508, and a container 510. In various embodiments, atwo-electrode configuration, with an anode 512 and a cathode 514, isemployed. Anode 512 may be a mesh made of high-purity aluminum wires.Cathode 514 may be a back electrode of a partially-finished siliconsolar cell ready for front electrode metallization. In variousembodiments, cathode 514 may comprise an aluminum back electrode.

FIG. 3 shows a portion of the process of light-induced aluminum platingon a partially-processed silicon solar cell 600, in accordance withvarious embodiments. Partially-processed silicon solar cell 600 maycomprise p-n junction 610 and cathode 514 on p-type base 604 ofpartially-processed silicon solar cell 600. In various embodiments,light-induced aluminum plating is performed on a silicon substrate. Thesilicon substrate may comprise n-type emitter 602 of partially-processedsilicon solar cell 600. The silicon substrate may further comprise asilicon nitride layer 622 disposed on n-type emitter 602 ofpartially-processed silicon solar cell 600. Silicon nitride layer 622may be patterned and/or otherwise treated to remove silicon nitride fromsome areas, thereby exposing an n-type surface 626, which comprises thesilicon disposed at the junction between silicon nitride layer 622 andn-type emitter 602. In various embodiments, a light source 504 ofsuitable wavelength excites electrons of partially-processed siliconsolar cell 600, and these photo-generated electrons create electron-holepairs in partially-processed silicon solar cell 600. In variousembodiments, p-n junction 610 drives photo-generated electrons towardn-type emitter 602 and holes toward p-type base 604 in response tostimulation from light source 504. On the areas of n-type surface 626where silicon nitride is removed, the photo-generated electrons mayreduce aluminum ions 627 in the ionic liquid 530, resulting indeposition of metallic aluminum 628 on n-type surface 626. Aluminumdeposition may not occur on silicon nitride layer 622.

In various exemplary embodiments, ionic liquid 530 comprises aluminum.In an exemplary embodiment ionic liquid 530 comprises a commerciallyavailable ionic liquid, 1-ethyl-3-methylimidazolium tetrachloroaluminate(EMIm-AlCl₄). However, any suitable organic halide or other ionic liquidmay be utilized in light-induced aluminum plating system 500. In variousembodiments, ionic liquid 530 comprises EMIm-AlCl₄ and anhydrous AlCl₃powder. Ionic liquid 530 may be prepared in a dry nitrogen box toprevent the ionic liquid from absorbing moisture. However, in variousembodiments, ionic liquid may be prepared in a dry beaker at roomtemperature.

In various embodiments, the molar ratio of AlCl₃ to EMIm-AlCl₄ in ionicliquid 530 is more than zero, but less than one, making ionic liquid 530a Lewis acid and/or enabling electroplating of aluminum. In variousembodiments, the molar ratio of AlCl₃ to EMIm-AlCl₄ in ionic liquid 530is about 0.5. However, any moral ratio suitable for aluminumelectroplating may be utilized. After mixing AlCl₃ and EMIm-AlCl₄, aprebake step may be performed in which ionic liquid 530 is heated toabout 120° C. for about one hour to drive out the residual moisture init. In various embodiments, however, any suitable prebake time and/orprebake temperature may be utilized. In various embodiments, the colorof prepared ionic liquid is yellow to brown.

In an exemplary embodiment, light source 504 for light-induced aluminumplating is selected in response to the transmission spectrum of theprepared ionic liquid and the absorption spectrum of silicon. Thetransmission spectrum of ionic liquid 530, as shown in FIG. 4, wasmeasured with a spectrophotometer in a quartz cuvette. The absorptionspectrum of a partially-processed silicon solar cell, as shown in FIG.5, was measured with an external quantum efficiency tester. The combinedabsorption spectrum, as illustrated in FIG. 6, was determined by theproduct of the transmission spectrum of ionic liquid 530 and theabsorption spectrum of a partially-processed silicon solar cell atdifferent illumination wavelengths. In various embodiments, a wavelengthof light source 504 of between about 600 nm and about 1,000 nm resultsin about 80% absorption of light by the silicon solar cell. In variousembodiments, light-induced aluminum plating system 500 comprises a lightsource 504 having a wavelength of between about 600 nm and about 1,000nm, corresponding to visible red light and/or infrared light.

Light-induced aluminum plating system 500 may comprise light-emittingdiodes (LEDs) with a single wavelength and/or lamps with a broad rangeof wavelength between about 600 nm and about 1,000 nm. However,light-induced aluminum plating system 500 may comprise a light source504 having any suitable wavelength or wavelengths. In an exemplaryembodiment, light source 504 comprises red LEDs with a wavelength ofabout 620 nm. A LED array may be built with an area larger than the sizeof the partially-processed silicon solar cell 600 to be plated withaluminum. The LED array may be placed beneath container 510, andcontainer 510 may be transparent to the wavelength used, such that lightmay be transmitted through container 510, through ionic liquid 530, andtowards partially-processed silicon solar cell 600. However, in variousembodiments, light source 504 may be placed in any suitable portion oflight-induced aluminum plating system 500.

In various embodiments, container 510 prevents ionic liquid 530 fromabsorbing moisture from the air. The ambient air in container 510 may bean inert gas such as nitrogen. Container 510 and/or at least someportion of light-induced aluminum plating system 500 may be disposed ina vacuum. However, in various embodiments, light-induced aluminumplating system 500 may be disposed in open air. When light-inducedaluminum plating occurs in open air, ionic liquid 530 may heated to atemperature of between about 90° C. and about 200° C., to reducemoisture absorption by ionic liquid 530.

In an exemplary embodiment and with reference again to FIG. 3, cathode514 comprises a partially-processed silicon solar cell 600 which lacks afront finger electrode. Partially-processed silicon solar cell 600 maybe monocrystalline or multicrystalline. An appropriate pattern may becreated in silicon nitride layer 622 of partially-processed siliconsolar cell 600. The pattern may be created by laser ablation and mayexpose a silicon n-type surface 626 disposed beneath the silicon nitridelayer 622. In various embodiments, a patterned silicon nitride layer maycomprise openings of 10×0.5 mm².

Before plating, the patterned silicon nitride layer and/or the n-typesurface 626 may be cleaned. In various embodiments, the cleaning mayimprove the efficiency and/or efficacy of direct aluminum deposition onsilicon. An exemplary cleaning procedure is as follows: apartially-processed silicon cell with a patterned front silicon nitridelayer and/or exposed n-type surface is dipped in an aqueous solution ofhydrofluoric acid for less than one minute to remove native oxide in thepattern. The hydrofluoric acid may comprise a concentration of betweenabout 1% and about 7%. In various embodiments, the concentration ofhydrofluoric acid comprises 2%. In various embodiments, theconcentration of hydrofluoric acid comprises 5%. The partially-processedsilicon cell may be dipped in hydrofluoric acid for between about 15seconds and about 45 seconds. In various embodiments, thepartially-processed silicon cell is dipped in hydrofluoric acid forabout 30 seconds. The partially-processed silicon cell is then immersedinto an aqueous solution of sodium hydroxide for less than a minute toetch off a thin layer of damaged and/or contaminated silicon from thepatterned n-type surface. In various embodiments, the sodium hydroxideconcentration comprises 3%; however, any suitable concentration ofsodium hydroxide may be used. The partially-processed silicon cell maybe dipped in sodium hydroxide for between about 1 second and about 30seconds. In various embodiments, the partially-processed silicon cell isdipped in sodium hydroxide for about 15 seconds. The sodium hydroxideetch step is desirably controlled to prevent over-etch, i.e., to preventremoval of too much silicon from the surface. In various embodiments,the partially-processed silicon cell may be dipped a second time inhydrofluoric acid for less than about 45 seconds. In variousembodiments, aluminum back electrode 514 is not brought into contactwith hydrofluoric acid or sodium hydroxide during the cleaning process.In various embodiments, other suitable cleaning methods are used.

Partially-processed silicon solar cell 600 may be placed in container510 and oriented so the light shines on n-type surface 626 ofpartially-processed silicon solar cell 600. N-type surface 626 may be incontact with ionic liquid 530. During light-induced plating, aluminumdeposits onto n-type surface 626 to form a finger electrode 120 (withmomentary reference to FIG. 1c ).

With reference again to FIGS. 2a and 2b , light-induced aluminum platingmay be carried out in a two-electrode configuration with anode 512 andcathode 514. A mesh made of high-purity aluminum wire may be used asanode 512. In various embodiments, anode 512 is a sacrificial anode. Invarious embodiments, anode 512 is dissolved during light-inducedaluminum plating and may be periodically replaced. Anode 512 may becleaned by a short dip in an aqueous solution of hydrochloric acid,followed by a rinse in deionized water. In various embodiments, theconcentration of hydrochloric acid is 37%. The positive terminal of adirect-current power supply 508 may be connected to anode 512, andcathode 514 may be connected to the negative terminal of thedirect-current power supply. In various embodiments, cathode 514comprises a back side aluminum layer of partially-processed siliconsolar cell 600.

Cathode 514 may be isolated from ionic liquid 530, leaving only siliconnitride layer 622 and/or n-type surface 626 at least partially immersedin ionic liquid 530. In various embodiments and with reference to FIG.2b , a frame 540 may be used to at least partially immersepartially-processed silicon solar cell 600 in ionic liquid 530. Invarious embodiments, the frame comprises a lower carrier 542 and anupper carrier 544. Lower carrier 542 may be configured to surround anouter perimeter of partially-processed silicon solar cell 600. Lowercarrier 542 may comprise an inner flange and may define a centralaperture such that partially-processed silicon solar cell 600 may beseated on the inner flange with n-type surface 626 generally disposedwithin and/or above the central aperture. In various embodiments, uppercarrier 544 is configured to be coupled to at least one of lower carrier542 and partially-processed silicon solar cell 600. Upper carrier 544may be configured to create a liquid-tight seal betweenpartially-processed silicon solar cell 600 and the inner flange. Invarious embodiments, frame 540 is configured to allow ionic liquid 530to partially or fully contact n-type surface 626 while preventingcontact between ionic liquid 530 and other portions ofpartially-processed silicon solar cell 600, including cathode 514.

In various embodiments, light emitted from light source 504 passesthrough ionic liquid 530 and shines on n-type surface 626. Thetemperature of ionic liquid 530 may be raised and maintained at betweenabout room temperature and about 150° C. during the plating process. Thetemperature of ionic liquid 530 may be raised and/or maintained by heatsource 506. In various embodiments, heat source 506 comprises heat tapedisposed on an outer surface of container 510. However, heat source 506may comprise any suitable means of heating and/or maintaining anelevated temperature of ionic liquid 530 disposed in container 510. Invarious embodiments, the temperature of ionic liquid 530 during theplating process is between about 20° C. and about 150° C. In variousembodiments, the temperature of ionic liquid 530 during the platingprocess is about 25° C. In various embodiments, the temperature of ionicliquid 530 during the plating process is about 60° C. As demonstrated inFIGS. 7a -8 d, inclusive, thickness of aluminum deposits may increase asionic liquid temperature increases.

In response to applying light to n-type surface 626 and to applying asuitable voltage or current between anode 512 and cathode 514, aluminumdeposition on n-type surface 626 begins. In various embodiments, platingcan be carried out under a constant current, a pulse current, a constantvoltage, and/or a pulse voltage between anode 512 and cathode 514. Invarious embodiments, the current does not exceed the photo-generatedcurrent of the partially-processed silicon solar cell 600. Thephoto-generated current may be light-induced and/or may be determined bythe intensity of light emitted by light source 504.

Aluminum deposits on partially-processed silicon solar cell generated inaccordance with principles of the present disclosure have beencharacterized. FIG. 7 is a scanning electron microscopy image oflight-induced aluminum deposits on an n-type silicon cell with a backemitter. Large grains of aluminum are observed. In various embodimentsand with reference to FIG. 8a , energy dispersive x-ray spectroscopyreveals only aluminum on the aluminum/silicon stack. In variousembodiments and with reference to FIG. 8b , energy dispersive x-rayspectroscopy of light-induced aluminum deposits plated at 25° C.displays a strong peak at 1.49 keV verifying that the deposit is largelyaluminum. The weak peaks of oxygen and chlorine are residues from theionic liquid. The absence of any silicon peak indicates that thealuminum deposit completely covers the silicon surface, i.e. thealuminum deposit is continuous on the n-type surface.

The thickness of aluminum deposits on silicon solar cells was measuredby a profilometer. FIG. 8c shows the average thickness at differentplating temperatures. Although the plating current and time were keptconsistent, the average thickness and its variation increased withtemperature. The resistance of the aluminum deposits was measured with afour-probe method. With the aluminum deposit dimensions of 10×0.5 mm²,the resistivity of the aluminum deposits was determined. FIG. 8d showsthe resistivity of the aluminum deposits as a function of platingtemperature. The lowest average resistivity is about 4×10⁻⁶ Ohm-cm at25° C., which is about 1.5 times that of bulk aluminum. It is lower thanthe resistivity of 7×10⁻⁶ Ohm-cm by conventional aluminumelectroplating. Based on resistance measurements and the thickness ofthe aluminum deposit, the resistivity of as-deposited aluminum may beabout 5×10⁻⁶ Ohm-cm. In various embodiments, the resistivity ofas-deposited aluminum is at least as low as 4×10⁻⁶ Ohm-cm. These valuesare lower than that of screen-printed silver and qualify light-inducedaluminum as a substitute for screen-printed silver in silicon solarcells.

Although a sacrificial aluminum anode is used in various exemplaryembodiments, the concentration of aluminum in the ionic liquid may stilldecrease as the plating process goes on. When the plating current is toolarge, the anode dissolution rate may fail to catch up with thedeposition rate of aluminum on the cathode leading to depletion ofaluminum in the ionic liquid. If an abnormally low current is observed,additional AlCl₃ may be added to the ionic liquid to maintain thealuminum concentration therein. The ionic liquid itself can be reusedfor many runs of aluminum deposition if it is kept under vacuum or inertgas, or at a temperature of about 100° C. or greater, with a more orless constant aluminum concentration.

Silicon Solar Cells With a Light-Induced Aluminum Electrode

Principles of the present disclosure contemplate examples of siliconsolar cells which are compatible with light-induced aluminum as thefront finger electrode. The back-side aluminum electrode in these solarcells remains screen-printed. Some of these silicon solar cells andtheir fabrication processes are schematically illustrated in FIG. 9,FIG. 10, and FIG. 11.

A common commercial silicon solar cell is the p-type aluminumback-surface field cell as shown in FIG. 9a . It features a p-typesilicon wafer, an n-type front emitter, and an aluminum-dopedback-surface field. The fabrication process for this solar cell with alight-induced aluminum front electrode is schematically shown in FIG. 9b. It starts with a p-type silicon wafer with an appropriate boronconcentration. After surface texturing and cleaning, the diffusion onthe front side of the wafer forms an n+ layer with an appropriateconcentration and an appropriate depth of phosphorus, which is theemitter of the solar cell. A layer of silicon nitride is deposited onthe front side of the wafer after diffusion by plasma-enhanced chemicalvapor deposition. Aluminum metallization on the back side is performedfirst. It is realized by screen printing of aluminum. After the backaluminum electrode is formed and before the front finger electrode isfabricated, the silicon wafer is heated to a suitable temperature, forexample between 700° C. and 900° C., to fuse aluminum into silicon. Itforms a layer of high aluminum concentration on the back side of thesilicon wafer. After the high-temperature firing, the front side siliconnitride layer is patterned for the front finger electrode. Thepatterning can be realized by laser ablation, lithography or any othersuitable patterning technique. The partially-processed solar cell isthen placed in a light-induced aluminum plating system 500 to fabricatethe front aluminum electrode by light-induced plating, as describedherein. Finally the solar cell is annealed again at a moderatetemperature between 100° C. and 500° C. to complete the fabricationprocess. In various embodiments, annealing the solar cell may decreaseresistivity of the deposited aluminum. In various embodiments, theannealing may occur for between about thirty second and five minutes.However, the annealing may occur at any suitable temperature and for anysuitable time.

FIG. 10a illustrates an n-type back-emitter cell with a light-inducedaluminum front electrode. FIG. 10b illustrates a fabrication process foran n-type back-emitter cell with a light-induced aluminum frontelectrode. An n-type silicon wafer with an appropriate phosphorusconcentration is textured and cleaned. The diffusion on the front sideof the wafer then forms a n+ layer with a proper concentration and anappropriate depth of phosphorus, which is the front surface field of thecell. A layer of silicon nitride is deposited on the front side of thewafer after diffusion by plasma-enhanced chemical vapor deposition.Aluminum metallization on the back side is performed first by screenprinting. After the back side aluminum electrode is formed and beforethe front finger electrode is fabricated, the silicon wafer is heated toa temperature between 700° C. and 900° C. to fuse aluminum into silicon.It forms a layer of high aluminum concentration on the back side of thesilicon wafer, which is the p+ emitter of the cell. After thehigh-temperature firing, the front silicon nitride layer is patternedfor the front finger electrode. The patterning can be realized by laserablation, lithography or any other suitable patterning technique.Light-induced plating of aluminum is then performed on the front side ofthe wafer, as described above. Finally the cell is heated to a moderatetemperature between 100° C. and 500° C. for the formation of the frontelectrical contact.

It will be appreciated that the light-induced aluminum plating processdisclosed herein also works for high-efficiency cells such aspassivated-emitter rear contact (PERC) cells. As an example, FIG. 11aillustrates a p-type PERC cell with a light-induced aluminum frontelectrode and FIG. 11b illustrates a fabrication process for such acell. The starting wafer for this cell is p-type. After cleaning andsurface texturing, phosphorus is diffused into the front side of thewafer to form an n+ layer as the emitter of the cell. A silicon nitridelayer is deposited by plasma-enhanced chemical vapor deposition on thefront side of the wafer, i.e. the phosphorus-diffused side. On the backside of the wafer, an aluminum oxide layer is deposited by atomic layerdeposition. A silicon nitride layer follows the aluminum oxide layer byplasma-enhanced chemical vapor deposition.

In various embodiments, the back side aluminum electrode of a siliconsolar cell is formed by a method of laser annealing, in which analuminum layer is screen-printed onto the aluminum oxide/silicon nitridestack on the back side of the wafer. After heating and drying thescreen-printed aluminum layer, a laser is employed to locally heat theback side of the wafer through the aluminum layer. The laser annealingmay allow the aluminum layer to penetrate the aluminum oxide/siliconnitride stack and form a p+ region of high aluminum concentration underthe laser spot. An array of p+ regions may be created by moving thelaser spot across the back surface, which provides localized p+ emitterfor the cell.

In various embodiments, the back side aluminum electrode of a siliconsolar cell is formed by a method of patterning, in which a patterningtechnique (for example, lithography or laser ablation) is employed tocreate openings in the aluminum oxide/silicon nitride stack. An aluminumlayer is then screen-printed on the patterned aluminum oxide/siliconnitride stack. The wafer is then fired between 700° C. and 900° C. toform localized p+ emitter in the cell.

Finally the front silicon nitride layer on the n-type emitter, ispatterned by either laser ablation or lithography to expose an n-typesurface. Light-induced aluminum plating is then performed on thepatterned silicon nitride layer or n-type surface. A final anneal at amoderate temperature between 100° C. and 500° C. completes solar cellfabrication.

Capping Layer for Light-Induced Aluminum

Principles of the present disclosure also contemplate a capping layerfor the light-induced aluminum electrode, as shown in FIG. 1c . Thecapping layer may comprise zinc, tin, or any other suitable material.The capping layer may improve solderability of the light-inducedaluminum electrode. A solderable capping layer may be desirable for thealuminum electrode. In various embodiments, a zinc capping layer ispreferable because its price is lower, its melting point is higher, andits resistance is lower than other materials.

In various embodiments, the capping layer is fabricated on an aluminumelectrode by conventional electroplating and/or light-induced plating.An exemplary plating solution contains 30 g/L zinc chloride and 200 g/Lpotassium chloride with a pH value of about 5.5. The plating is carriedout with two electrodes, an anode and a cathode. The anode is an inertmetal such as titanium and the cathode is a silicon cell with alight-induced aluminum front electrode. Conventional plating of zinc maybe performed with a constant current of about 10 mA/cm² and at roomtemperature. For light-induced plating of zinc, a system similar tothose described herein may be used, and a heat sources and/or enclosurefor vacuum or inert gas may be omitted from such a system. The thicknessof the capping layer may be between 1 and 3 micrometers. The cappinglayer may be fabricated directly after light-induced aluminum plating,and the final anneal may be carried out after zinc plating.

EXAMPLE 1

An ionic liquid was prepared by adding anhydrous aluminum chloride(AlCl₃) powder (99%, Aldrich) to 1-ethyl-3-methylimidazoliumtetrachloroaluminate ((EMIm)AlCl₄) (≥95%, Aldrich) in a beaker on a hotplate in a nitrogen box to drive out any moisture in the ionic liquid.The molar ratio between AlCl₃ and (EMIm)AlCl₄ was 0.5 to keep it a Lewisacid for Al plating. The solution was stirred to dissolve all the AlCl₃power.

The prepared ionic liquid was transferred into a glass container, asillustrated in FIG. 2b . The tank was wrapped in a heating tape andplaced on a frame with a round hole in the center. An array oflight-emitting diodes (LEDs) with a wavelength of 620 nm shed light fromthe bottom of the container. A high-purity aluminum mesh was placed infront of the silicon sample as the anode, which was dissolved duringplating to supply the depleted aluminum ions to the ionic liquid.

The sample was a silicon p-n junction. It had a 75-nm silicon nitride(SiN_(x)) layer covering the n-type emitter and screen-printed aluminumon the p-type base. The SiN_(x) layer was patterned with openings of10×0.5 mm² by laser ablation where silicon n-type surface was exposed.The sample was cleaned in 2% hydrofluoric acid (HF) for 30 seconds andthen immersed in 3% sodium hydroxide (NaOH) for 15 seconds at roomtemperature to remove laser damage. With a final HF dip, the sample wasplaced on the sample holder with the pattern facing down and contactingthe ionic liquid. The backside aluminum on the sample served as thecathode for plating. The voltage applied between anode and cathode wascontrolled by a Gamry Reference 3000 potentiostat to achieve a platingcurrent of about 40 mA/cm². The plating temperature was between 25° C.and 70° C.

The photo-generated electrons in the sample were driven to the n-typeemitter by the p-n junction. The Al₂Cl₇ ⁻ ions in the ionic liquid arereduced by the photo-generated electrons and form metallic aluminumdeposit on the silicon n-type surface according to:

  4Al₂Cl₇⁻ + 3e⁻ ↔ Al + 7AlC??indicates text missing or illegible when filed

The applied voltage between anode and cathode was selected to moreeffectively extract the photo-generated electrons to the n-type emittersurface. The plating current became practically zero when the light wasturned off.

After plating, the sample was rinsed with methanol (CH₃OH) and distilledwater before being blown dry with nitrogen. The morphology andcomposition of the aluminum deposits were characterized by scanningelectron microscopy (SEM) and energy dispersive X-ray spectroscopy(EDX). Profilometry was used to measure the thickness of the aluminumdeposits. The resistance of the aluminum deposits was measured with afour-probe method which eliminates the effect of contact resistance.

A method 700 for light-induced electroplating of aluminum directly ontoa silicon substrate is disclosed herein. In various embodiments, themethod comprises preparing an ionic liquid comprising aluminum chloride(AlCl₃) and an organic halide (Step 701), placing the silicon substrateinto the ionic liquid (Step 704), illuminating the silicon substrate(Step 705), and depositing aluminum onto the silicon substrate via alight-induced electroplating process (Step 706), wherein thelight-induced electroplating process utilizes an applied current thatdoes not exceed a photo-generated current generated by the illumination.

In various embodiments, the method may comprise cleaning the siliconsubstrate with at least one of hydrogen fluoride, hydrogen chloride,hydrogen peroxide, sodium hydroxide, potassium hydroxide, or ammoniumhydroxide (Step 703). In various embodiments, the method may comprisepatterning the silicon nitride layer of a partially-processed siliconsolar cell to expose the silicon n-type surface (Step 702). In variousembodiments, the method may comprise cleaning the deposited aluminumwith deionized water (Step 707). In various embodiments, the method maycomprise annealing the deposited aluminum and the silicon substrate toreduce a resistivity of the deposited aluminum (Step 708).

While the principles of this disclosure have been shown in variousembodiments, many modifications of structure, arrangements, proportions,elements, materials and components, used in practice, which areparticularly adapted for a specific environment and operatingrequirements may be used without departing from the principles and scopeof this disclosure. These and other changes or modifications areintended to be included within the scope of the present disclosure.

The present disclosure has been described with reference to variousembodiments.

However, one of ordinary skill in the art appreciates that variousmodifications and changes can be made without departing from the scopeof the present disclosure. Accordingly, the specification is to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope of thepresent disclosure. Likewise, benefits, other advantages, and solutionsto problems have been described above with regard to variousembodiments. However, benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential feature or element.

When a phrase similar to “at least one of A, B, or C” or “at least oneof A, B, and C” is used in the claims, the phrase is intended to meanany of the following: (1) at least one of A; (2) at least one of B; (3)at least one of C; (4) at least one of A and at least one of B; (5) atleast one of B and at least one of C; (6) at least one of A and at leastone of C; or (7) at least one of A, at least one of B, and at least oneof C.

As used herein, the terms “comprises,” “comprising,” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises a list ofelements does not include only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Also, as used herein, the terms “coupled,”“coupling,” or any other variation thereof, are intended to cover aphysical connection, an electrical connection, a magnetic connection, anoptical connection, a communicative connection, a functional connection,and/or any other connection.

What is claimed is:
 1. A method for light-induced electroplating ofaluminum directly onto a silicon substrate, the method comprising:preparing an ionic liquid comprising aluminum chloride (AlCl₃) and anorganic halide; placing the silicon substrate into the ionic liquid,wherein the surface of the silicon substrate is n-type and does notcomprise a seed layer; illuminating the silicon substrate with a lightsource comprising at least one light-emitting diode with a wavelength ofabout 620 nanometers, the illumination passing through the ionic liquid;and depositing aluminum onto the surface of the silicon substrate via alight-induced electroplating process, wherein the light-inducedelectroplating process utilizes an applied current that does not exceeda photo-generated current generated by the illumination.
 2. The methodof claim 1, further comprising cleaning the silicon substrate with atleast one of hydrogen fluoride, hydrogen chloride, hydrogen peroxide,sodium hydroxide, potassium hydroxide, or ammonium hydroxide.
 3. Themethod of claim 1, further comprising patterning a partially-processedsilicon solar cell to expose the silicon substrate.
 4. The method ofclaim 3, wherein the patterning comprises at least one of laser ablationor lithography.
 5. The method of claim 1, further comprising cleaningthe deposited aluminum with deionized water.
 6. The method of claim 1,further comprising annealing the deposited aluminum and the siliconsubstrate to reduce a resistivity of the deposited aluminum.
 7. Themethod of claim 1, wherein the organic halide is1-ethyl-3-methylimidazolium tetrachloraluminate (EMIm-AlCl₄).
 8. Themethod of claim 1, wherein the light-induced electroplating processutilizes a two-electrode electrolyzer.
 9. The method of claim 8,wherein, in the two-electrode electrolyzer, an anode comprises analuminum wire mesh, and a cathode comprises the silicon substrate. 10.The method of claim 9, wherein the light-induced electroplating processcomprises applying a voltage between the anode and the cathode toachieve a current of between 30 milliamps per centimeter squared and 50milliamps per centimeter squared.
 11. The method of claim 1, wherein thedepositing is performed with the ionic liquid at a temperature ofbetween 20 degrees Celsius and 150 degrees Celsius.
 12. The method ofclaim 1, wherein the depositing is performed with the ionic liquid at atemperature between 100 degrees Celsius and 200 degrees Celsius.
 13. Themethod of claim 1, wherein the depositing occurs in an inert ambientatmosphere.
 14. The method of claim 1, wherein the light source furtherprovides illumination comprising another wavelength of between 600nanometers and 1000 nanometers.
 15. The method of claim 1, wherein theionic liquid is disposed in a container having a transparent bottom, andwherein the illumination is provided by light emitting diodes disposedbelow the bottom of the container.
 16. The method of claim 1, whereinthe depositing is performed without use of a secondary tank for anoxidation reaction.
 17. A method for processing a silicon solar cell,the method comprising: preparing an ionic liquid comprising aluminumchloride (AlCl₃) and 1-ethyl-3-methylimidazolium tetrachloraluminate(EMIm-AlCl₄); patterning a partially-processed silicon solar cell toexpose an n-type surface of a silicon substrate; cleaning the n-typesurface with at least one of hydrogen fluoride, hydrogen chloride,hydrogen peroxide, sodium hydroxide, potassium hydroxide, or ammoniumhydroxide; bringing the n-type surface into contact with the ionicliquid, wherein the n-type surface does not comprise a seed layer;illuminating the n-type surface, wherein the illumination passes throughthe ionic liquid and wherein the illumination is generated at leastpartially by a light source comprising at least one light-emitting diodewith a wavelength of about 620 nanometers; depositing aluminum onto thesilicon substrate via a light-induced electroplating process, whereinthe light-induced electroplating process comprises applying a currentbetween an aluminum back electrode of the partially-processed siliconsolar cell and an aluminum mesh disposed in the ionic liquid; cleaningthe deposited aluminum with deionized water; and annealing the depositedaluminum on the silicon substrate to reduce the resistivity of theelectroplated aluminum.
 18. The method of claim 17, wherein theillumination is generated at least partially by a second light sourcehaving a wavelength of between 600 nanometers and 1000 nanometers.